Method for defining reference magnetizations in layer systems

ABSTRACT

The invention is used in the field of materials engineering and relates to a method for defining reference magnetizations which could be used, for example, in magnetic sensor technology components. The object of the present invention is to disclose a method for defining reference magnetizations in layer systems, whereby the reference directions can be selected as desired with regard to number and spatial direction. The object is attained through a method for defining reference magnetizations in layer systems in which at least one layer system is produced by geometrically structuring a hard-magnetic and/or soft-magnetic layer and by applying the hard-magnetic and/or soft magnetic layer to at least one antiferromagnetic layer before, during or after a single-stage or multi-stage thermal treatment, whereby the temperature is increased at least to a temperature greater than the coupling temperature and the layer system is cooled afterwards.

FIELD OF APPLICATION OF THE INVENTION

[0001] The invention is used in the field of materials engineering andrelates to a method for defining reference magnetizations that could beused, for example, in components used in magnetic sensor technology orspin electronics, such as, e.g., in GMR sensors or MRAM storage cells.

PRIOR ART

[0002] The use of exchange coupling between ferromagnets andantiferromagnets (AFM) or artificial antiferromagnets (AAF) to securemagnetizations in magnetic layer systems is known.

[0003] In most cases components in magnetic sensor technology or spinelectronics require a fixed reference magnetization direction. To thisend the magnetic coupling to a so-called “anchor layer” is frequentlyutilized. This anchor layer can comprise a hard magnet, a natural orartificial antiferromagnet. The magnetization direction of theferromagnetic layer is spatially fixed through the exchange couplingbetween ferromagnet and anchor layer.

[0004] This anchor layer itself must likewise be magnetically aligned.To this end, depending on the material properties of the anchor layer,until now the following processes have been used:

[0005] Layer deposition in an external magnetic field

[0006] Thermal aftertreatment in an external magnetic field

[0007] Field cooling after local laser irradiation

[0008] In all three cases an effective field cooling is carried out,i.e., the ferromagnet/anchor layer system is transferred from a stateabove the critical coupling temperature (blocking temperature T_(B))with applied magnetic field to a coupled state. Thus a homogenousmagnetization of the ferromagnetic layer forced by the magnetic field isimpressed by direct exchange coupling into the spin configuration of theantiferromagnetic layer. For external field strengths below the couplingfield strength, the adjusted homogenous magnetization of theferromagnetic layer is maintained and thus serves as referencemagnetization.

[0009] Of the processes listed above, only the last one is able tolocally change reference magnetizations in the area of the focus of alaser beam.

[0010] The disadvantages of the known processes are that, with theexception of the laser process, it is impossible to realize severalreference directions in any directions to one another at the same time.This is necessary for the functionality of more complex magnetoelectriccomponents, such as, e.g., angle sensors.

[0011] And finally subsequent process steps for aligning the AFM arenecessary and complicated and only to a certain extent compatible withthe microminiaturization.

[0012] It is further known that in soft-magnetic layer elements themagnetization is orientated along the element edges in order to avoidstray fields. The magnetic flux thus provides a closed configuration. Asvan den Berg discovered, the magnetization also remains parallel to thenearest element edge in the interior of the element. In points that havethe same distance to two element edges, the different magnetic areascollide. A state thus arises with homogenous magnetic domains that areseparated by domain walls.

[0013] It is known that elements separated from one another by asufficiently small distance interact with one another via their strayarea. In order to achieve a state that is favorable in energy terms,neighboring elements adopt magnetization configurations that are closeto a closed magnetic flux and cause only small stray fields.

DESCRIPTION OF THE INVENTION

[0014] The object of the current invention is to disclose a method fordefining reference magnetizations in layer systems, whereby thereference directions can be selected as desired with regard to numberand spatial direction.

[0015] The object is attained through the invention disclosed in theclaims. Further developments are the subject matter of the subclaims.

[0016] With the method according to the invention for defining referencemagnetizations in layer systems, at least one hard-magnetic and/orsoft-magnetic layer is produced by geometrically structuring ahard-magnetic and/or soft-magnetic layer and by bringing thehard-magnetic and/or soft-magnetic layer into direct contact with atleast one antiferromagnetic layer before, during or after a single-stageor multi-stage thermal treatment. The thermal treatment is therebyconducted with a temperature increase of at least up to a temperaturegreater than the coupling temperature. Afterwards the layer system iscooled.

[0017] Advantageously the layer system is cooled without the applicationof a magnetic field after the thermal treatment so that the demagnetizedstate or the residual state impresses itself undisturbed as a referencemagnetization.

[0018] It is further advantageous to cool the layer system after thethermal treatment in an external magnetic field, in order to impress ademagnetized or residual state changed by the field as a referencemagnetization.

[0019] Advantageously the layers are produced with lateral expansions inthe micrometer and nanometer range and layer thicknesses in thenanometer range.

[0020] It is also advantageous if several layers with the same ordifferent composition are heated to a temperature greater than thecoupling temperature and are afterwards cooled without a magnetic field.

[0021] According to the invention the method for defining referencemagnetizations is used in magnetoresistive sensor elements ormagnetoresistive switching elements based on an anisotropicmagnetoresistance or on a giant magnetoresistance or on a tunnelmagnetoresistance or on a spin injection magnetoresistance or in activemagnetoelectronic components based on a giant magnetoresistance or on atunnel magnetoresistance or on a spin injection magnetoresistance.

[0022] With the method according to the invention, first a hard-magneticand/or soft-magnetic layer is geometrically structured. This can occurwith processes known from microelectronics, such as, e.g., lithographicprocesses. Through this geometric structuring, the form, number andarrangement of these geometric elements to one another are determined.This process step has a decisive influence on the magnetizationdirection of the hard-magnetic and/or soft-magnetic layer, sinceaccording to the principle discovered by van den Berg, the magnetizationdirection within the respective form is determined through the selectionof the geometric form. Within a form, domains form whose magnetizationis orientated parallel to the nearest edge. Alternatively, the strayfield interaction of neighboring elements can be used for the formationof desired domain patterns.

[0023] Thus through the number, form, and/or arrangement to one another,as many reference directions as desired and as many different referencedirections as desired can be produced in a layer system.

[0024] After the geometric structuring, heating to a temperature greaterthan the coupling temperature enables the magnetization configurationsto be adjusted in the hard-magnetic and/or soft-magnetic layer that isfree due to the temperature increase, corresponding to the domainelements. In the subsequent cooling without application of a magneticfield, the antiferromagnetic layer takes over the magnetizationconfiguration of the hard-magnetic and/or soft-magnetic layer. Thus thelayer system features a uniform magnetization configuration.

[0025] It is also possible according to the method according to theinvention for the hard-magnetic and/or soft-magnetic layer alone to besubjected to the thermal treatment and to be applied to anantiferromagnetic layer only during or after the cooling. Here too theantiferromagnetic layer takes over the magnetization configuration ofthe hard-magnetic and/or soft-magnetic layer.

[0026] If the hard-magnetic and/or soft-magnetic layer is applied or canbe applied only after the production of the antiferromagnetic layer, itsstructuring can be carried out, for example, by an alternating maskprocess or lithographically controlled ion etching.

[0027] When the antiferromagnetic layer is present during the thermaltreatment, the magnetization of the antiferromagnet is determined not byan applied magnetic field, but by the magnetization of theexchange-coupled ferromagnetic layer.

[0028] According to the invention, it is also possible to apply amagnetic field during the thermal treatment. A decaying alternatingmagnetic field can promote the adjustment of the pattern therebyaccording to the description of van den Berg. A sufficiently strongconstant magnetic field can bring about residual magnetization states ina targeted manner.

[0029] A further advantage of the method according to the invention isthat the domain patterns of the hard-magnetic and/or soft-magnetic layerare maintained even at higher temperatures and thus the method is alsocompatible with the temperature treatment for the production of anantiferromagnetic state, such as, e.g., with PtMn and similarsubstances.

[0030] Furthermore it is advantageous that the reference magnetizationsdefined by the method according to the invention can be regenerated(self-healing). This can only be realized by a re-heating of themulti-layer structure above the coupling temperature. Thus destroyedmagnetizations above the coupling temperature are reestablished andafter cooling, can again serve as reference magnetizations.

[0031] With the miniaturization of magnetoelectronic components, themethod according to the invention can readily be used, since it isapplicable over a wide scaling range. A reliable defining of thereference magnetization can be achieved in particular in thesubmicrometer range.

THE BEST WAY TO CARRY OUT THE INVENTION

[0032] The invention is described in more detail below on the basis ofseveral exemplary embodiments. Thereby

[0033]FIG. 1 Shows a typical magnetization configuration of aferromagnetic layer and an antiferromagnetic layer

[0034] a) before a thermal treatment

[0035] b) at T>T_(B), where T_(B)-coupling temperature

[0036] c) after a thermal treatment

[0037] (the layers are drawn separately to give a better view) and

[0038]FIG. 2 Shows a Kerr microscope image of 4 ellipse-like structuredelements, whereby the magnetization shows downwards in the blackelements and upwards in the white elements.

EXAMPLE 1

[0039] For a 360° GMR angle sensor, reference magnetizations at rightangles to one another are needed. For this purpose a 10 nm-thick FeMnlayer is first deposited on silicon as an anchor layer, and a 100nm-thick ferromagnetic Ni₈₁Fe₁₉ layer is deposited thereon. Squares withan edge length of 24 μm are structured with the aid of lithographictechniques. The ferromagnetic layer must be completely removed outsidethe structure. Then the thermal treatment takes place at 200° C. Whenthe temperature of 200° C. is reached, the sample is demagnetized in adecaying magnetic field of maximum amplitude of 1 kA/cm and then cooledto room temperature without the action of a magnetic field. The layersystem now shows a stable magnetization configuration according to FIG.1.

EXAMPLE 2

[0040] Magnetoresistive magnetic field sensors are advantageouslyembodied in a Wheatstone bridge circuit. In order to realize the signalsof the individual elements of the Wheatstone bridge that are inverse toone another, reference magnetizations antiparallel to one another areneeded. A double layer comprising 10 nm FeMn and 100 nm Ni₈₁Fe₁₉ issputtered onto a silicon substrate. A homogenous magnetic field of thestrength 240 A/cm is applied during the layer deposition. In thesubsequent lithography step, 4 elements of an ellipse-like form with thelateral dimensions of 100 μm×20 μm are structured. The elements areorientated parallel to one another and to the field direction during thelayer deposition and lie adjacent to one another at a distance of 30 μm.Now the thermal treatment takes place at 200° C. When the temperature of200° C. is reached, the sample is demagnetized in a decaying field ofmaximum amplitude of 1 kA/cm orientated diagonally to the element axisand is then cooled to room temperature without the action of a magneticfield. The layer system now shows a stable magnetization configuration,as shown in FIG. 2.

1. Method for defining reference magnetizations in layer systems,whereby at least one layer system is produced by geometricallystructuring a hard-magnetic and/or soft-magnetic layer and by applyingthe hard-magnetic and/or soft-magnetic layer to at least oneantiferromagnetic layer before, during or after a single-stage ormulti-stage thermal treatment, whereby the temperature is increased atleast to a temperature greater than the coupling temperature, andafterwards the layer system is cooled.
 2. Method according to claim 1,whereby the layer system is cooled without the application of a magneticfield.
 3. Method according to claim 1, whereby the layer system iscooled with an applied magnetic field, whereby, depending on the desiredreference magnetization, the magnetization configuration of thehard-magnetic and/or soft-magnetic layer and/or the antiferromagneticlayer is changed.
 4. Method according to claim 1, whereby a magneticfield is impressed into the layer.
 5. Method according to claim 1,whereby layers are produced with lateral dimensions in the micrometerand nanometer range and layer thicknesses in the nanometer range. 6.Method according to claim 1, whereby several layers with the same ordifferent composition are heated to a temperature greater than thecoupling temperature and are afterwards cooled without a magnetic field.7. Method according to claim 1, whereby square, rectangular, triangular,circular structurings or forms derived therefrom are generated. 8.Method according to claim 1, whereby the thermal treatment is carriedout until complete penetration is achieved.
 9. Method according to claim1, whereby the geometric structuring is carried out two- orthree-dimensionally.
 10. Method according to claim 1, whereby ahard-magnetic and/or soft-magnetic layer is geometrically structured andapplied to an antiferromagnetic layer, afterwards a single-stage thermaltreatment is carried out with cooling.
 11. Use of the method fordefining reference magnetizations in magnetoresistive sensor elements ormagnetoresistive switching elements based on an anisotropicmagnetoresistance or on a giant magnetoresistance or on a tunnelmagnetoresistance or on a spin injection resistance or in activemagnetoelectronic components based on a giant magnetoresistance or on atunnel magnetoresistance or on a spin injection resistance.